Lack of energy is a serious threat to the prosperity of many developed states that have access to or plan to use nuclear power. The paper describes a concept for solving two major problems related to these conditions, namely safe disposal of spent reactor fuel and generation of electric energy or hot fluid by use of heat produced by the disposed waste. The challenge in storing spent nuclear fuel can be met with by installing such highly radioactive waste in deep boreholes with fuel rods encapsulated in canisters of copper-lined iron or titanium. Electric energy can be generated by utilizing the accumulated heat in the same rock mass by pumping up hot water or clayey mud from series of deep holes bored parallel and between corresponding holes with nuclear waste. The amounts of heat in each of the hot-water holes overlap and raise the initial rock temperature at 1,500-3,000 m depth to about 80-90oC after some 50 years and to 70-80oC in 500 years, after which the temperature in the hot-fluid holes goes down successively to the initial value 60-70oC in about 500 years. If these holes are subsequently deepened from 3,000 to 5,000 meters, utilization of the hot fluid can continue for another 500 years.

Effective diffusion tensors were computed for tracer diffusion through compacted water-saturated bentonite at two distinct scales by combining random microstructures and microstructures obtained by Transmission Electron Microscopy. The original micrographs have been thresholded by considering four distinct phases at the mesoscopic scale, and image analysis techniques have been employed in order to characterize the morphology and in particular the texture anisotropy of grains, clay gels and macrovoids. The Homogenization of Periodic Media approach employed is based on the local description ion diffusion at both the microscopic level of clay platelets (assuming variable diffusivity and ion sorption) and the mesoscopic level of clay aggregates and macropores. The local problems were successively solved using randon and TEM-based numerical microstructures in order to investigate the contribution to macroscopic diffusion of soft gels, dense gels and macrovoids under various configurations. Comparisons were made with existing diffusion data for montmorillonite and natural bentonite, and a particular attention was given to the anisotropy of the macroscopic diffusion tensor in connection with morphological characteristics of the underlying microstructure. The effect of clay matrix diffusivity on the magnitude and orientation of the macroscopic diffusion tensor has been investigated, and the principal axes of diffusion have been compared with the index of fabric orientation. Computations were also performed on microstructures of increasing size in order to investigate the model implicit assumption of existence of a Representative Elementary Volume.

One of the major challenges of high-level nuclear waste (HLW) isolation in deep boreholes is to anticipate the maturation behaviour of swelling clay when the waste, surrounded by dense clay encased in perforated tubes, is submerged into the borehole mud. The ultimate homogeneity of this clay seal acting as a barrier is expected to stabilize the borehole and to prevent possible leakage of radioactivity. In this study, a numerical model for predicting the maturation of the clay barrier has been developed. In the model, the water transport is controlled by the differences in the suction potential and the permeability. The model is able to simulate the maturation process, both the expeditious water transport and the clay migration into the surrounding mud, from beginning to end. Results from laboratory tests of the clay maturation were compared with the predictions made by the model. They are in good agreement, but refinement is proposed by taking the impact of the tube perforation on the maturation rate into more consideration. The proposed numerical model will also be useful in selecting a suitable design for clay barriers in HLW boreholes. Different combinations of clay dimensions and initial densities of mud and dense clay can thus be studied to determine the final homogenization and the end densities.

Global interim storage of HLW has reached a level that requires large extension of the storage capacity, which puts pressure on regulatory authorities and national parliaments for finding and applying ways of safe disposal of such waste. An important option is to use very dense natural expandable clay for isolating spent nuclear fuel in boreholes where it will be exposed to high temperature for hundreds to a few thousand years. The clay must be placeable and homogeneous and be able to sustain significant shear strain and temperatures up to 150oC without leaking or losing its ductile behaviour and self-healing potential. In this document the long-term function of such seals, which have the form of dense smectite blocks and soft smectite mud surrounding the containers/canisters will be described with respect to the impact of degrading physical/chemical mechanisms. Focus is on clay barriers for isolating spent nuclear fuel in up to 3-4 km deep boreholes but aspects are also provided on disposal in mined repositories at a few hundred m depth below the ground surface.

In either case the dense clay surrounding the waste containers will expand and enclose them, and consolidate the surrounding mud, which successively becomes denser, while the dense clay seal softens until its swelling pressure and the pressure of the mud is the same. 

The clay seals in deep boreholes used for disposal of spent nuclear fuel consist of a central core of dense expandable clay in perforated tubes (“supercontainers”) submerged in clay mud according to a concept termed VDH. In the lower parts of 3-4 km deep boreholes these tubes, made of copper, Navy Bronze, titanium or steel, host canisters lined with highly compacted expandable clay. In the upper parts of the holes the same type of supercontainers with no waste but with dense smectite clay blocks make up a primary barrier to possibly released radionuclides. A second barrier is the heaviness of the strongly saline groundwater at depth, which prevents such water to reach high up to the biosphere. The role of the mud is to save the supercontainers from touching the borehole walls when being placed, and to seal voids in the borehole walls with clay. The dense clay and soft clay mud will interact physically and ultimately become a homogeneous silicified clay body. Creep strain in the rock causes the deposition holes to con, which increases the radial pressure on the clay seals and thereby eliminates flow and diffusive migration of possibly contaminated porewater from the deployment part to move to the ground surface.

At present two ideologies appear to govern the international selection of concepts for isolation of radionuclides escaped from High-Level Radioactive Waste (HLW) stored underground: “shallow” burial in mined repositories in crystalline rock, and deep geologic disposal in holes bored in crystalline or sedimentary rock, making use of metal canisters isolated from the rock by concrete or dense expansive clay. The present paper describes disposal in smectite clay for delaying or preventing radionuclides from reaching the biosphere when they can still cause great havoc. This is achieved by utilizing the great waste-isolating capacity of the expansive clay through its high hydrophilic capacity and large specific surface area, providing low porosity and limited interconnectivity of the voids, which both makes such clay low-permeable and operating with a very low through-diffusion rate of anionic species like iodine, and of some cationic radionuclides. The expandability of such clay means that it can swell and undergo self-healing in case of microstructural contraction caused by heating. The mechanisms involved in permeation and ion exchange are described based on conceptual microstructural models and their theoretical analogies. Stress/strain phenomena involved in saturation with fluids, desiccation, shearing under deviatoric conditions, and creep strain under stable conditions or at failure are described as well. Longevity matters are given limited space.

An alternative but still untested approach is to dispose of highly radioactive waste in very deep boreholes, a concept being considered in the UK and the US. One version of this concept known as Very-Deep-Hole (VDH) storage proposes drilling of up to 4 km deep holes and placing in a series of stacked super-containers sealed by a combination of dense clay and concrete. In this case the risk of losing clay material by dispersion and erosion where the holes intersect fracture zones would be eliminated by casting concrete of a new type. VDH containment relies on the use of copper or Navy Bronze tubes filled with dense clay in the upper parts of the holes and with clay-embedded canisters with waste in the lower parts. The maturation and isolating function of the clay seals in the holes and their chemical interaction with concrete and waste canisters are considered and assessed. The work has focused on the mechanical testing and microscope investigations of the clay/concrete. Hydrothermally treated clay and concrete, exposed to 20-150°C, which represents the conditions over the entire length of a real VDH, have been tested. The recorded impact of strong heating showed stiffening and some reduction in hydraulic conductivity of the clay and concrete.

Most of the proposed techniques for chemical shielding of radioactive waste implies use of smectite clay, which degrades according to a well-known scheme. Since such “buffer” clay has to serve for many thousands of years, the mechanisms in the unavoidable long-term degradation process must be understood and accounted for as described in the paper. In addition to conversion of smectite to non-expandable minerals cementation by precipitation of siliceous matter created in the degradation process is of concern since it can reduce the self-sealing capacity of desiccated or mechanically damaged clay.

As for isolation of high-level radioactive waste by use of smectite clay it serves very well also for hindering radionuclides from low- and intermediate-level waste to contaminate groundwater. It can be used for minimizing groundwater flow through and along waste packages and for providing them with ductile embedment for eliminating the risk of damage caused by displacements in host rock or concrete vaults. The clay can have the form of liners placed and compacted on site over vaults constructed on the ground surface, or consist of compacted blocks of clay granules that are tightly placed around waste packages in underground drifts and rooms. In either case the initially incompletely water saturated clay will swell in conjunction with water uptake until tight contact with the confining medium has been established. The clay seals must be sufficiently dense to fulfill criteria set with respect to hydraulic conductivity and swelling capacity, paying due attention to the salt content in the porewater. Their physical and chemical stabilities must be acceptable in short- and long-term perspectives, which is a few hundred years for most low-level wastes up to tens of thousands of years for long-lived waste. 

Countries using nuclear energy tend to favour disposal of Low-Level and Intermediate-Level radioactive waste in mined repositories consisting of series of tunnels or drifts connected to disposal tunnels at a few hundred meters depth. Abandoned mines can serve as repositories for such waste and the present study indicates that this would be possible also for High-Level Waste in the form of spent reactor fuel. The technique implies encapsulation of such waste in metal canisters surrounded by densely compacted smectite clay in relatively shallow mined repositories as well as in very deep bored holes. Intermediate-Level radioactive waste can be disposed of in caverns in the form of packages of metal containers cast in low-pH concrete and embedded in dense smectitic clay.

Landfill site`s selection represents a complicated process due to the large number of variables to be adopted. In this study, an arid area (Babylon Governorate as a case study) was selected. It is located in the middle region of Iraq. In this area, the  landfills do not satisfy the required  international criteria.  Fifteen  of the  most significant criterion were selected for this purpose. For suitable weight for each criterion, the multi criteria decision making (MCDM) methods were applied. These methods are AHP and RSW. In the GIS software 10.5, the raster maps of the chosen criterion were arranged and analysed. The method of change detection was implemented to determine the matching pixels and non-matching pixels. The final results showed that there are two candidate locations for landfills for each district in the governorate (ten sites). The areas of the selected sites were sufficient to contain the cumulative quantity of solid waste from 2020 until 2030.